1. Field of the Invention
The present invention pertains to a sealing device for retaining a coupling fluid such as degassed water within an ultrasound transducer housing.
2. Description of the Prior Art
Ultrasound transducers require a coupling medium to connect the transducer to a patient in order to minimize the reflection and refraction of ultrasound waves when those waves cross a border between two materials of similar acoustic properties. One of the biggest issues in coupling transducers to a patient either for a diagnostic ultrasound device, or a therapeutic ultrasound device, is the presence of air. Coupling agents are used to eliminate air bubbles between the transducer and the patient. For diagnostic purposes, mineral oils, hydro-gels and even water can be used to couple a transducer to a patient. In therapeutic procedures the coupling agent should be more strictly controlled so that even minute air bubbles are eliminated.
In high intensity focused ultrasound (HIFU) procedures the need to couple the transducer to the patient often includes a means of cooling the face of the transducer, or cooling a patient's skin, with a medium that will pass ultrasound energy with little or no attenuation or adverse effect. Typically this medium is water, sometimes with antibacterial additives, held within a transmission cavity with a cap or membrane, and through which the ultrasound energy passes.
One major issue with such a system arises from bubble formation caused by dissolved gasses being drawn out of solution. These bubbles provide an impedance mismatch to the ultrasound energy, causing scattering and localized heating, leading to observed effects such as reduced effectiveness of therapy, the destruction of the cap or seal, or patient hyperemia.
Atmospheric water for example, contains approximately 8.5 PPM (parts per million) O2, and 14.5 PPM N2 as well as other dissolved gasses. Using dissolved oxygen (DO) as an indicator, it is possible to determine the relative contents of other gasses, CO2, CO, N2, etc. . . This can be done using the partial pressure values of the other gases. Reducing the concentration of DO (and other gases) inhibits the incidence of cavitation. However for high intensity focused ultrasound (HIFU) procedures, the optimal dissolved gas content is highly dependent on the treatment being performed, and the type of ultrasound instruments being utilized. To date, we are aware of no treatise clearly defining the operable boundaries of DO and other dissolved gases in HIFU operations.
The common method used by the industry is to prepare the fluid by passing it through a filtration and de-ionization process to remove impurities and particulates that may precipitate out, contaminate or provide nucleation sites for bubbles. The coupling fluid is then degassed to some minimum level before introduced into the system. Typically degassing is performed by bulk cavitation under a vacuum or boiling at atmospheric or sub atmospheric pressure and then sealing the degassed fluid in a container.
In a completely sealed system the dissolved gas content will remain constant, but as described below the gas content will strive to meet equilibrium with the partial pressure of the local atmospheric conditions. During short procedures or low power ultrasound procedures the re-gas rate is usually slow enough not to cause problems. In longer procedures and/or at higher powers, the probability that re-dissolved gas will be drawn into the fluid, and subsequently interfere with ultrasound transmission, goes up considerably since it is impossible to prevent gas diffusing through the system lining, joints and seals without investing in prohibitively expensive parts and materials.
The methods by which gasses come out of solution or enter the cooling system are various, some examples of the more common range from pressure changes within the cooling system caused by physical restriction to atmospheric conditions. Local pressure changes such as rectified diffusion from HIFU or temperature changes will bring gas out of solution as will displacement of the partial pressure of one gas by another, or by material leaching. Other methods by which gas may enter the system include diffusion through the tubing, seals and structure of the cooling system in the same way a balloon deflates, trapping micro bubbles within the surface structure and pockets of the cooling system, chemical reactions between materials in the cooling system, or as a by product of bacterial growth within the cooling system.
Precautions such as using low permeability materials for the tubing are regularly employed, but even with such precautions, the re-gas rate can become a major issue. Other methods used to reduce the effects of re-gassing include the introduction of surfactants or wetting agents to prevent bubble formation, using larger volumes of fluids, and the use of hydrophilic and/or hydrophobic polymers such as polyvinylpyrolidone (PVP). Experimental testing has shown these provide only a short term solution.
Numerous examples in the prior art show differing solutions to the problems of dealing with coupling HIFU transducers to a patient as well as providing an apparatus for degassing a fluid. However there has been thus far nothing demonstrating the feasibility or utility of an in line degassing mechanism combined with a HIFU therapy system during an actual medical procedure or application. The use of an inline degasser during a procedure mandates the use of a transducer housing having a cavity where the cooling/coupling fluid may circulate around the transducer. To prevent the coupling fluid from escaping the cavity and to minimize gas from entering the cavity, a seal is needed.
The inability of the prior art to maintain a controlled dissolved gas content in a cooling fluid over a prolonged procedure acts as a forced limitation to prolonged HIFU therapy.
Thus there remains a need for a seal capable of retaining a degassed coupling fluid for use in a HIFU procedure within a cavity containing a HIFU transducer.